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United States Patent |
5,196,800
|
Graff
,   et al.
|
March 23, 1993
|
Apparatus and method for non-contact measurement of the edge sharpness
of a knife
Abstract
Non-contact measurement of the sharpness of the cutting edge of a slitting
or chopping knife is made repeatably placing a capacitance sensor probe
symmetrically over the cutting edge at a predetermined distance (nominal
offset) from the calibrated sharp edge of the knife and measuring the
capacitance to derive a measurement that varies with knife wear
(increasing knife edge radius). A novel sensor holder allowing for
repeatable precision placement of the sensor.
Inventors:
|
Graff; Ernest A. (Ontario, NY);
Podhorecki; Mathew J. (Clearwater, FL)
|
Assignee:
|
Eastman Kodak Company (Rochester, NY)
|
Appl. No.:
|
759027 |
Filed:
|
September 13, 1991 |
Current U.S. Class: |
324/662; 73/104; 340/680 |
Intern'l Class: |
G01R 027/26 |
Field of Search: |
324/661,662,690
73/104
340/680
|
References Cited
U.S. Patent Documents
2472994 | Jun., 1949 | Vars | 73/104.
|
3641431 | Feb., 1972 | Pigage et al. | 324/662.
|
4620281 | Oct., 1986 | Thompson et al. | 364/475.
|
4885530 | Dec., 1989 | Mayer et al. | 73/104.
|
4958129 | Sep., 1990 | Poduje et al. | 324/690.
|
5101165 | Mar., 1992 | Rickards | 324/690.
|
Primary Examiner: Harvey; Jack B.
Assistant Examiner: Regan; Maura K.
Attorney, Agent or Firm: Boos, Jr.; Francis H.
Claims
What is claimed is:
1. Apparatus for measuring the sharpness of the cutting edge of a knife,
the cutting edge being formed at the line of intersection between two
converging side cutting surfaces defining an acute angle between the
surfaces, the apparatus comprising:
a capacitance sensor probe having an active sensor area, the sensor area
having a principal reference plane;
means for holding said active sensor area repeatably at a predetermined
nominal offset from said cutting edge with the principal reference plane
of the active sensor area substantially normal to a plane bisecting the
acute angle formed between said side cutting surfaces;
and means for coupling the central sensor lamination of said active sensor
area to a capacitance measuring-instrument,
whereby changes, in sharpness of the cutting edge are measured by changes
in capacitance between the cutting edge and the sensor active area in
successive measurements.
2. The apparatus of claim 1 in which said holding means is further adapted
to hold said active sensor area substantially symmetrically positioned
laterally with respect to said plane
3. The apparatus of claim 1 wherein said capacitance sensor probe comprises
a laminated sensor.
4. The apparatus of claim 3 wherein said sensor probe includes a central
capacitance sensor lamination having an elongated active sensor area and
wherein said holder means is adapted to hold an elongated dimension of
said active sensor area in alignment with a lengthwise dimension of said
cutting edge.
5. The apparatus of claim 1 in which said holder includes a first plurality
of knife contacting projections straddling said active sensor area and
adapted to contact said side cutting surfaces adjacent said cutting edge
for holding said active sensor area at said predetermined nominal offset.
6. The apparatus of claim 3 in which said sensor probe includes a central
capacitance sensor lamination having an elongated active sensor area and
said holder includes a first plurality of knife contacting projections
straddling said active surface area and adapted to contact said side
cutting surfaces adjacent said cutting edge for holding an elongated
dimension of said sensor area in substantial alignment with a lengthwise
dimension of said cutting edge.
7. The apparatus of claim 1, 5 or 6 in which said holder includes means for
contacting the knife at a position remote from said side cutting surfaces
for holding said principal reference plane of the active sensor area
perpendicular to said plane bisecting the acute angle between the cutting
surfaces.
8. A method of measuring the degree of sharpness of an elongated cutting
edge of a knife the cutting edge being at an intersection between two
converging side cutting surfaces of the knife forming an acute angle
therebetween, the method comprising the steps of:
positioning a shielded capacitance sensor probe having an active surface
area at a predetermined nominal offset from the cutting edge of the
knife,;
orienting a principal reference plane of said active sensor area
orthogonally of a plane bisecting said acute angle;
and measuring the capacitance between said active sensor area and the
cutting edge to determine sharpness of the cutting edge.
9. The method of claim 8 wherein said active sensor area comprises an
elongated area, the method further including the step of orienting an
elongated dimension of the active surface area substantially in alignment
with the cutting edge.
Description
FIELD OF INVENTION
This invention relates to the field of measuring the sharpness of knife
cutting edges, specifically industrial knives, using capacitance sensing
as the measurement technique.
BACKGROUND
Several methods are known for measuring the sharpness of knives, for
example, of the type used on slitters, choppers and other similar
machines. Such machines are typically used in cutting webs, such as
plastic sheet, film supports, cloth material and the like. One of the
known methods is to take a plastic mold of the knife edge. When the mold
has cured, it is removed from the knife and sliced with a razor. The knife
profile is then viewed under a microscope to get a visual indication of
the sharpness of the cutting edge of the knife. This requires that a
specially trained technician make a subjective determination of the knife
sharpness and the point at which the knife should be re-sharpened.
Another method is to use a small piece of lead on the end of a punch-like
device. A technician places the lead directly over the cutting edge of the
knife and strikes the punch, causing a small indentation in the lead
corresponding to the knife profile. This lead model is then cut and the
knife profile examined in a manner similar to the plastic mold method. In
addition to the reliance on the subjective judgment of a trained
technician, other problems with this method are that the results are
dependent on the angle that the punch is held over the knife edge and the
force with which the punch is struck. Additionally, there is the
possibility that the knife edge can be damaged in the process.
Non-contact capacitance measurement techniques, in general, are also known.
Examples are found in U.S. Pat. Nos. 3,641,431-Pigage et al and
4,620,281-Thompson et al. The '431 patent involves measurement of radial
trueness of the position of cutter blades retained in an indexer of a gear
cutting machine. A capacitance probe is mounted in a holder positioned
adjacent the blades and measured capacitance of the air gap between the
probe and the face or edge of the cutter blade is used to determine radial
trueness of the blade position in the indexer. This arrangement does not
measure sharpness of the cutting edge of the cutter blades, however. In
the '281 patent, the condition of a cutting tool is sensed during the
cutting operation by means of a capacitive sensor mounted on the cutting
tool. The capacitive sensor is used to measure the distance between the
tool sensor and the freshly cut surface of the workpiece being cut.
Reduction in the measured distance provides an indication of nose wear of
the cutting tool. Such an arrangement does not provide a direct
measurement of knife sharpness. In addition, it assumes the ability to
measure capacitance between the capacitive probe and a workpiece and, as
such, is not suitable for use in directly measuring the sharpness of
knives used in slitter and chopper equipment used for cutting webs and the
like.
It is therefore an object of the present invention to provide a method and
apparatus for measuring knife sharpness directly and in a non-contact
manner.
It is a further object of the invention to provide non-contact knife
sharpness measurement which is simple in structure and operation and
provides repeatable measurement with a high degree of accuracy.
It is another object of the invention to provide non-contact knife
sharpness measurement that directly measures the sharpness of the knife
without relying on subjective judgment of a technician.
SUMMARY OF THE INVENTION
Thus, in accordance with the invention, apparatus is provided for measuring
the sharpness of the cutting edge of a knife, wherein the cutting edge is
formed nominally at a line of intersection between two converging cutting
side surfaces defining an acute angle therebetween. The measuring
apparatus comprises a capacitance sensor probe including an active sensor
area having a principal reference plane useful in defining the spatial
orientation of the active sensor area. The apparatus further comprises
means for holding the active sensor area of the probe repeatably at a
predetermined nominal offset from the cutting edge with the principal
reference plane of the active sensor area substantially normal to a plane
bisecting the acute angle formed between the side cutting surfaces of the
knife and with the active sensor area substantially symmetrically
positioned laterally with respect to such plane. The apparatus finally
includes means for coupling the sensor probe to a capacitance measuring
instrument whereby changes in sharpness of the cutting edge can be
measured by changes in capacitance between the cutting edge and the sensor
active area in successive measurements.
In the method of the invention, the degree of sharpness is measured of an
elongated cutting edge of a knife wherein the cutting edge is generally at
an intersection between two converging cutting faces of the knife which
form an acute angle therebetween. The method of the invention comprises
the steps of positioning a capacitance sensor probe having an active
sensor area at a predetermined nominal offset from the cutting edge of the
knife; orienting a principal reference plane of the active sensor area of
the probe perpendicular to a plane bisecting the aforesaid acute angle.
Preferably, the active sensor area has an elongate dimension in the
principal reference plane and the method would then include the step of
orienting the elongated dimension substantially in alignment with the
lengthwise dimension of the cutting edge. Finally, the method includes the
step of measuring the capacitance between the active sensor area of the
probe and the cutting edge to determine wear of the cutting edge from a
predetermined sharp condition.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a perspective view from the bottom of knife sharpness measuring
apparatus of the invention.
FIG. 2 is a side elevation view of the FIG. 1 measuring apparatus.
FIG. 3 is a schematic end view of a capacitive sensor probe useful in the
apparatus of FIG. 1.
FIGS. 4a-4c are plan views of laminar components useful in the fabrication
of the sensor probe of FIG. 3.
FIG. 5 is a perspective view of a coaxial connector useful in the
fabrication of the sensor probe of the apparatus of FIG. 1.
FIG. 6 is schematic representation of a sensor probe and knife edge useful
in describing the present invention.
FIG. 7 is a graph of measurement characteristics of the apparatus of FIG. 1
.
DESCRIPTION
Referring to FIGS. 1 and 2, knife sharpness measuring apparatus of the
invention includes a capacitance sensor probe 10 and a holder 12 for
positioning the sensor probe on the knife 13 to be measured. Knife 13 as
shown in FIG. 2 includes a cutting edge 13a formed nominally at a line of
intersection between converging side cutting surfaces 13b. and 13c which
define an acute angle .alpha. therebetween. As shown in FIGS. 3 and 4a-4c,
in a presently preferred form of the invention, sensor probe 10 is
comprised of a plurality of sandwiched metallic laminations 15, 20 and 22
each of which is formed on a corresponding insulative, adhesive substrate
15a, 20c and 22c. These substrates serve as bonding agents when the
laminates are sandwiched together to form the completed sensor assembly
and as electrical insulation between adjacent laminates. In the
fabrication process, the central lamination 15, shown in FIG. 4c, is
preferably made of copper and is etched by known techniques to form a
capacitance sensor 16 terminating at one end in an active sensor area 16a
perpendicular to the sheet of the drawing and at the other end in an
electrical contact terminal 16b. A shield area 18 is formed on laminate 15
to provide an active lateral shield along the edges of capacitance sensor
16. In FIG. 4b, laminate 20, of which two are required and preferably also
made of copper, is etched as shown to provide outer active shield areas
20a to shield the face areas of capacitance sensor 16. An isolated central
area 20b is formed which will eventually be electrically connected to
contact terminal 16b. A pair of laminates 22, preferably made of stainless
steel, are etched as shown in FIG. 4a to form an outer ground shield for
sensor probe 10 which will be electrically in contact with knife 13 via
holder 12 and which serves to isolate the sensor 16 and shields 20 from
the surrounding environment. After bonding the laminations together as
shown in FIG. 3, holes 23 and 24 are drilled to provide electrical
connection to central capacitance sensor 16 and to the active shield areas
18 and 20a. For this purpose, a central capacitance sensor connector wire
26 and peripheral active shield connector wires 27 of a threaded coaxial
connector 25 (FIG. 5) are inserted into holes 23 and 24, respectively, and
soldered in place. Terminals 27 are connected on the reverse side of
sensor 16 to a shield cap 28.
The active sensor area 16a at the end of sensor probe 10 is preferably
created by precision cutting with a diamond cutter along line 29 (FIG.
4a). When cut, the active sensor area 16a. is then defined by a principal
reference plane which corresponds in FIG. 3 to the plane of the drawing
sheet. In FIG. 4c, the principal reference plane is perpendicular to the
plane of the drawing sheet and coincident with the elongated axis or
dimension of area 16a represented by line 29.
Referring again to FIGS. 1 and 2, holder 12 functions as a gauge body to
hold the active capacitance sensor area 16a repeatably in proper
orientation relative to the cutting edge of the knife 13 for capacitance
measurement purposes. More specifically, holder 12 is provided with an
internal slot 30 and set screw 31 for receiving and fixedly holding the
laminated sensor probe 10 in the holder. Outer knife contact projections
32a, 32b and center knife contact projection 32c straddle the capacitance
probe active sensor area 16a and are adapted to contact knife surfaces
13b, 13c simultaneously to position the active sensor area 16b at a
predetermined nominal offset distance from the knife cutting edge and to
substantially align the elongated axis of sensor area 16a with the
lengthwise dimension of the cutting edge of the knife 13. Although any
number of contacts may be used, a three point contact is preferred for the
inherent stability it provides to the gauge body when positioned on the
knife cutting surfaces 13b, 13c. Holder 12 is further provided with an
additional pair of contact projections 33a and 33b spaced significantly
from the first set of contact projections 32a-32c for contacting knife 13
at a position remote for the cutting surfaces for holding the principal
reference plane of the sensor area 16a perpendicular to a plane 35
bisecting the acute angle .alpha. formed by the convergence of the cutting
surfaces 13b, 13c of the knife. Holder 12 is preferably made from an
electrically conductive material, such as stainless steel, so as provide
electrical contact between the ground plane represented by the knife and
the outer ground shields 22 of sensor 10.
It can be shown that when a capacitive probe, such as the active sensor
area 16a of sensor 16, is suitably placed over the cutting edge of knife
13, a capacitance measuring instrument connected to that probe with the
knife at ground potential will read a capacitive value that depends on the
geometry of the probe, the nominal distance from the probe active area to
the knife edge and the geometry of the knife edge in accordance with the
relationship:
C=KA/d
where:
C=capacitance
K=a constant
A=active measurement area
d=distance between the sensor and target (ground point).
Thus with the probe geometry fixed and a probe holder such as described
above designed to place the sensor active area repeatably over the knife
cutting edge in a fixed orientation, the only variable is the knife
geometry.
Referring to FIG. 6 the relationship of the active sensor area 16a to the
knife cutting edge is shown schematically with sensor area 16a positioned
at a nominal offset, x, from the actual line of intersection e of the
converging side surfaces 13b and 13c of the knife. The cutting edge 13a of
a theoretically perfectly sharpened would coincide with this line. In
actuality, a freshly sharpened knife edge would have some finite radius as
shown by edge 13a'. It can be seen that as the knife edge wears during
repeated cutting operations, the edge radius increases and the the average
distance from the sensor active area 16a to the worn knife edges 13a" and
13a'" correspondingly increases with a consequent decrease in capacitance
measurement. Thus, a probe repeatably placed over the cutting edge of
knife at a fixed nominal distance from the knife edge (nominal offset)
will have a capacitance value that will vary with changes in knife radius
caused by knife wear. FIG. 7 illustrates the manner in which measured
capacitance, in picofarads, varies as a function of knife radius.
In the performance of the method of the invention, sensor 10 is initially
placed over a knife edge of a known degree of sharpness and positioned at
a predetermined nominal offset from the cutting edge with the principal
plane of the sensor active area normal to the plane bisecting the acute
angle of the knife edge and with an elongated dimension of the active
sensor area aligned with the cutting edge. The capacitance of this
arrangement is measured to form a baseline measurement corresponding to a
"sharp" knife. Then as the knife is used in slitting or chopping
operations, the sensor is periodically re-placed in this same position
over the knife edge and capacitance measurement taken. The measurement can
then be compared to a pre-calibrated graph of the type shown in FIG. 7, or
else the capacitance reading can be digitized and fed to a microcomputer
having a lookup table in memory with appropriate values which can then be
converted to a visual readout of appropriate type, such as actual knife
radius or percent degree of sharpness.
It will be appreciated that the geometry of the active sensor area 16a may
be other than the elongated rectangle shown in FIG. 3. For example, an
oval, circle or diamond shaped active area may be used. However, at
present, the rectangular shape represents the presently preferred mode of
practicing the invention due at least in part to the fact that it lends
itself to easy fabrication of the sensor using the laminar fabrication
process described above. In addition, due to the fact that an ultra narrow
rectangle is more readily confined to the actual cutting edge, a more
sensitive measurement of an average capacitance along an extended length
of the knife cutting edge can be obtained.
The invention has been described in detail with particular reference to a
presently preferred embodiment, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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